Mg-incorporated sorbent for efficient removal of trace CO from H2 gas

Removal of trace CO impurities is an essential step in the utilization of Hydrogen as a clean energy source. While various solutions are currently employed to address this challenge, there is an urgent need to improve their efficiency. Here, we show that a bead-structured Mg, Cu, and Ce-based sorbent, Mg13CuCeOx, demonstrates superior removal capacity of trace CO from H2 with high stability. The incorporation of Mg boosts sorption performance by enhancing the porous structure and Cu+ surface area. Remarkably, compared to existing pelletized sorbents, Mg13CuCeOx exhibits 15.5 to 50 times greater equilibrium capacity under pressures below 10 Pa CO and 31 times longer breakthrough time in removing 50 ppm CO in H2. Energy-efficient oxidative regeneration using air at 120 °C allows its stable sorption performance over 20 cycles. Through in-situ DRIFTS analysis, we elucidate the reaction mechanism that Mg augments the surface OH groups, promoting the formation of bicarbonate and formate species. This study highlights the potential of MgCuCeOx sorbents in advancing the hydrogen economy by effectively removing trace CO from H2.

The solution was then stirred in an oil bath at 80°C for 5 h and dried at 90°C for 2 h.
The dried sample was ground and sieved (mesh size of 250-600 μm) to obtain a sample with a specific diameter range.The sample was then dried overnight at 110°C, resulting in a powder or spherical bead morphology depending on the precursor metal type and composition.Lastly, the resulting sample was calcined using a flow of 21% O2 (with an N2 balance, hereafter referred to as air for simplicity) by ramping the temperature to 450°C at a heating rate of 1°C min -1 and maintained for 10 h.The final samples were denoted as CuCeOx and MgαCuCeOx, with α indicating the weight percentage of Mg compared with the total metal.For comparison, pristine AC and AC-Cu + , which exhibited superior CO sorption performance in a previous study 1 , were prepared using the same method described in previous research.The pristine AC support was pre-treated at 700°C for 3 h under H2 flow.The pretreated AC was then impregnated with a solution of copper formate tetrahydrate and HCl, targeting a copper content of 4.26 mmol per 1 g of sorbent, and stirred at 60°C for 1 h.After impregnation, the sorbent was washed, filtered, and dried at 60°C overnight.The final AC-Cu + was prepared by activating the dried sorbent under N2 flow at 100 mL min -1 and 300°C for 3 h.

Characterization
The structural properties of the as-synthesized chemisorbent samples were assessed using N2 adsorption/desorption isotherms acquired at 77 K with an Autosorb IQ instrument (Quantachrome, version 5.21).To examine the morphology and elemental distribution of the chemisorbents, a combination of field-emission scanning electron microscopy (FE-SEM; JSM-7610F-Plus, JEOL, Ltd.) and transmission electron microscopy (TEM; JEM-ARM 200F (NEOARM), JEOL, Ltd.) was employed.Elemental mapping was performed utilizing an energy-dispersive X-ray spectrometer (EDS; X-MAX TSR, OXFORD Instruments).
The crystalline structure of the as-prepared chemisorbent samples was assessed using X-ray diffraction (XRD) analysis conducted with an Ultima IV diffractometer (Rigaku), operating at 40 kV and 100 mA with Cu-Kα radiation (λ = 1.54 Å).XRD patterns were recorded over a 2θ range of 10° to 100°, with scanning increments of 0.02°.To determine the metal composition of the samples, inductively coupled plasma optical emission spectrometry (ICP-OES; 5110, Agilent) was employed.
The temperature-programmed reduction using H2 (H2-TPR) was carried out using a ChemBET Pulsar TPR/TPD unit (Quantachrome).For this, 50 mg of a sorbent sample was loaded into a U-shaped quartz reactor and subjected to reduction by heating from ambient temperature to 800°C at a heating rate of 10°C min −1 under a 10% H2/Ar gas flow.A cold trap was employed to capture water produced during the experiment before entering the thermal conductivity detector (TCD).
The surface chemistry of the samples was characterized using XPS (Thermo Fisher Scientific) with monochromated Al Kα radiation as the excitation source.The Ce3d spectra were subjected to baseline subtraction, after which a well-resolved u'" peak was identified at around 915 eV in the spectrum.The positions of the remaining peaks were determined by referencing the binding energy shift relative to the u'" peak 2 .The Cu2p spectra comprising Cu2p 3/2 and Cu2p 1/2 peaks were deconvoluted into Cu + and Cu 2+ components 3 .Finally, the ratios of Ce 3+ to Ce species and Cu + to Cu species were calculated based on their respective peak areas.
Cu dispersion and the specific Cu surface area (SCu) were determined using selective N2O chemisorption experiments conducted at 50°C following a well-documented methodology 4,5 .Additionally, since Cu + is the active site for the sorption of CO on MgCuCeOx, the surface area of Cu + of the as-prepared sorbent (SCu+) was estimated by applying the surface Cu + ratio to SCu.Initially, the samples were reduced under a flow of 10% H2/Ar at 400°C for 1 h, and the H2 consumption was measured by integrating the peak area.The chemisorbent bed was then cooled to 50°C and flushed with He.The cooled chemisorbent was exposed to 10% N2O/He gas for 30 min before being returned to room temperature and purged with He.An additional H2-TPR cycle was performed and the corresponding H2 consumption was assessed.Cu dispersion (D) was calculated based on the H2 consumption after the total oxidation of the catalyst in the first H2-TPR run (X) and subsequent oxidation of surface Cu atoms in the second H2-TPR run (Y).Cu dispersion and SCu were determined using Supplementary Equations ( 1) and ( 2) respectively: where Nav is the Avogadro constant (6.02 × 10 23 mol -1 ), WCu indicates the Cu metal weight content determined using ICP-OES analysis, and ACu is the atomic weight of Cu (63.546 g mol - 1 ).The ratio of Cu + to Cu 2+ (M), obtained from the XPS analysis, was then used to calculate

CO sorption and desorption test
CO sorption isotherms were acquired using a commercial sorption analyzer (Autosorb IQ, Quantachrome, version 5.21) and the conventional static volumetric method under pressures of up to 1 kPa.Before analysis, the CuCeOx and MgCuCeOx samples underwent vacuum degassing at 280°C for 8 h to eliminate adsorbed impurities.
For the breakthrough experiment, the chemisorbent samples were subjected to pretreatment to eliminate impurities or emulate the conditions immediately after synthesis.AC was treated under a stream of He at 120°C for over 12 h, AC-Cu + under He at 200°C for 3 h, and MgαCuCeOx under air at 280°C for 30 min.The breakthrough apparatus (Supplementary Fig. 1) consisted of three mass flow controllers (MFCs; Bronkhorst Co.) to supply the gas, a resistance temperature detector (RTD, Pt 100 Ω) to control the temperature, a reactor with an internal diameter of 0.7 cm, and a CO-infrared analyzer with a detection limit of 0.02 ppm (CO-IR, Everise), details of which can be found in a previous study 1 .The MFCs were calibrated using a mass flow meter (Hastings Inc., USA) and a soap bubble flow meter (Supelco Co., USA).All experiments were conducted at 1 bar, 25°C, and a gas hourly space velocity (GHSV) of 935 h −1 .
The desorption behavior of Mg13CuCeOx was examined using CO temperatureprogrammed desorption (CO-TPD) with a Chem BET Pulsar TPR/TPD unit in conjunction with an online mass spectrometer (MS; HPR 20, Hiden Analytical Ltd.).A U-shaped quartz reactor was used to contain the sample (0.05 g).Following chemisorbent pretreatment under the same conditions as for the breakthrough experiment, the temperature was reduced to 25°C and the samples purged with He.The samples were then exposed to 1% CO (He balance) at a flow rate of 50 ml min −1 at 25°C for 1 h to reach saturation.A He purge was then conducted until no residual gas was detected by the MS.The regeneration of the chemisorbent samples was then conducted using either He or air under heating up to 800°C.
The cyclic CO uptake of Mg13CuCeOx was investigated using TGA (TGA 4000, Perkin Elmer) at 1 bar.Prior to each experiment, the Mg13CuCeOx sample was subjected to pretreatment at 450°C for 30 min under air or N2 (40 mL min −1

Table 2 .
Infrared band assignments for CO chemisorption on CuCeOx andMgCuCeOx.